I intend to determine the N-terminal amino acid sequence of an abnormal antibody polypeptide chain synthesized in vitro by translation of messenger RNA from a mouse myeloma tumor cell line. This abnormal chain contains the amino acid sequence corresponding to the constant region of normal antibody light chains but does not contain the sequence corresponding to the variable region. The abnormal chain seems to contain a short "leader" of extra amino acids N-terminal to the constant region, in the place where the variable region would ordinarily lie. I hope that the abnormal chain is coded by a constant-region half-gene in its germline state, before it has been joined to a variable-region half-gene to form the gene for a complete antibody chain. If so, the leader, which is the portion of the abnormal chain I hope to sequence, might correspond to the DNA sequence which is usually displaced by the variable-region half-gene when joining occurs. This sequence may help to elucidate the mechanism of joining. BIBLIOGRAPHIC REFERENCES: G.P. Smith, Evolution of repeated DNA sequences by unequal crossover. Science 191:528(1976) (b) Significance of Hybridization Kinetic Experiments for Theories of antibody diversity. Cold Spring Harbor Symposium on Quant. Bio. 41 (in press, 1976).